The present invention is related to communication networks which require reliable voice communication services over widely varying environmental conditions. In particular, the invention concerns the design of components for the processing of signals carrying voice information in the transmission and receiver systems of a communication network.
With the advent of low cost miniaturized hand sets for voice communication, particularly, those which rely upon wireless or satellite communication links, offering the opportunity for communication from widely diversified geographical locations or under widely varying conditions, there is a need for signal processing techniques and equipment that will ensure reliable transmission and reception of signals carrying voice information. The difficulty in addressing this need is compounded with the use of such equipment in mobile applications, such as those related to land-mobile vehicles, aeronautical and maritime. While reliable communication under such conditions is desirable for commercial or private purposes, there is a clear need for highly reliable communications related to military and law enforcement applications worldwide, and even those involving governmental functions, such as those related to the diplomatic efforts of personnel in embassies or other stations around the world.
Communication via two-way voice pagers, HF-VHF packet radio and the increasingly popular handsets would benefit from an improved voice communication technology. The need for such advance is particularly apparent for wireless mobile or satellite mobile channels. For mobile satellite channels, the quality of voice will depend on the type of channel that is used, for example, land mobile, maritime, or aeronautical. Among these channel types, the land mobile/satellite link is the most diversified since it can be established in environments having widely varying operational parameters.
On a land-mobile satellite link, the received signal level changes rapidly because of multipath fading. The signal level is also attenuated by shadowing due to the transmission link obstacles such as buildings, trees, foliage, and environment terrain. In the traditional full duplex links, fading in the forward link (in the direction of base station to the mobile) is independent of fading in the reverse link because separations between frequency bands in the forward channel and the reverse channel is more than the coherence bandwidth of the channel.
Conventionally, for a fixed channel symbol rate, the transmission voice quality can be improved by dynamically adapting the system parameters to the channel state conditions by varying one or a combination of the following parameters:
Voice codec bit rate PA1 Transmit power level PA1 Error control redundancy rate PA1 Modulation signal space
The variation of such conditions typically occurs through the use of a separate signaling channel, which must provide fast and reliable communication of control information between the transmitter and the receiver. The signaling channel may be in-band or out-of-band, and may involve an open loop communication (one-way without feedback as to the success or accuracy of a transmission) or a closed loop communication (two way with feedback).
"Gear Shifting" is a well known method to improve the voice quality over dynamically changing mobile channels where, for a given received C/N, voice quality (or voice codec bit rate) is traded off against the available E.sub.b /N.sub.0. The receiver and the transmitter "sense" the channel state and switch the transmitted bit rate to a lower or a higher rate according to variations in the link conditions. Switching from one state to another state is usually performed upon mutual agreement between the transmitter and the receiver through messages exchanged over this signaling channel.
A major drawback of the gear shifting technique and other well known adaptive voice transmission schemes is their dependency on the existence of a fast and reliable signaling channel, in-band or out-of-band. In a terrestrial network, where propagation delay is small, the closed loop signaling technique may be acceptable. However, when the communication link includes a satellite, the 540 ms delay encountered during the round trip over geostationary satellite links is considerably larger than the coherence bandwidth of the channel. For low earth orbiting satellites and intermediate circular orbiting satellites, the propagation delays are 20 ms and 132 Ms, respectively, which are again significantly larger than the fading frequency. The reliability of the signaling channel cannot be guaranteed under all channel conditions since the signaling channel might also be unreliable when the transmission link is in a deep fade or shadow.
Power control, involving control over transmit power level, is another technique for improving the voice quality over a dynamically changing channel environment. However, its effectiveness is limited by the satellite or base station EIRP in the forward direction, and the hand held battery power in the return direction. Moreover, a high powered transmitter causes extra interference to adjacent channels. Therefore, power control in conjunction with other adaptive techniques, based on tradeoffs among other system parameters, should out perform a system design which only uses power control. Existing approaches also employ a voice coder that operates at a fixed voice coding rate, and use prioritized channel protection.
Error control may be used for improving the quality by increasing the rate of redundancy for the transmitted signal. However, this technique requires adaptively changing the amount of redundant data which is being transmitted. This approach requires constant monitoring and feedback of the channel condition for purposes of varying the redundancy rate.
High level modulation schemes, i.e., trellis coded 8PSK modulation have been extensively analyzed for mobile applications. However, high level modulations are more sensitive to phase noise and other mobile channel impairments and require a larger link C/N for achieving a high bandwidth efficiency.
Even when transmission parameters are appropriately managed in order to improve voice quality, voice codecs may be selected to further enhance system performance. As an alternative to the conventional fixed rate voice codec, variable rate codecs using multi-mode coding have been used. In multi-mode coding, different voice coding algorithms and/or techniques are used for each of several different bit rates.
Where multi-mode coding was used, a different voice coder is employed for each rate. With this approach, each rate can have optimal voice quality, but higher hardware complexity is required. Further, switching between rates can introduce discontinuities. In addition, the multi-mode coding technique is sensitive to proper mode detection and control errors can result from degraded channels. The dependence upon a feedback or "reverse" channel control of the transmitted bit rate offers a further disadvantage.
Accordingly, it is an object of the present invention to provide a nested voice codec which transmits voice information at the highest bit rate and at the receive side a decoder that derives lower rate signals for lower quality voice communication.
It is a further object to provide a nested coding approach which permits the reduction of transmitted bit rate at any point in the transmission path.
It is yet another object of the present invention to provide a channel coding technique which is compatible with digital voice signals generated by a nested voice codec.
It is a further object of the present invention to provide a combination of channel decoder and nested voice decoder which operates to derive from a transmitted signal an intelligible voice communication.
It is an object of the present invention to provide a voice communication system which combines low rate speech coding with advanced channel and modulation techniques.
It is a further object of the invention to provide a voice communication system which provides automatic rate reduction under degraded conditions but without transmit side control and with high efficiency.
It is a further object of the invention to provide gradual speech degradation which retains connectivity under degraded transmission conditions.
It is yet another object of the present invention to provide a voice communication system, including a transmitter having a nested voice codec and compatible channel coding, in combination with a receiver having a nested voice decoder in combination with a channel decoding system.
It also is an object of the present invention to provide a receiver having a channel state estimator which selectively controls the voice decoder on the basis of detected channel parameters including carrier power, unique word correlator, bit error rate estimator and decoded bit reliability.